In many ways, a computer disk is much like an old fashioned record player. That is, most disks include a platter spun by a rotating spindle. A disk head, controlled by a disk head actuator, moves about the spinning platter reading or writing information in different platter locations. Thus, to read or write data, the disk head actuator moves the disk head relative to the platter to await the location of interest to spin underneath.
Disks vary widely in their characteristics. For example, disks differ in the number of bits they can store. Disk capacity is often measured in Megabytes or Gigabytes. Disks also vary in their density. That is, some disks offer more storage for their size than others.
The performance of a disk can be measured in a number of ways. For example, performance can be measured by bandwidth (e.g., bits or bytes per second) or throughput (e.g., IOPS (Input/Output requests per Second)). Another performance measurement known, as “data access density”, measures IOPS per gigabit.
Disk capacities continue to grow rapidly, doubling roughly every 18 months. While also growing, data access speeds have generally failed to keep pace, doubling only every 24 months. Thus, while disk capacities and throughput grow, the disparity between their growth implies that the data access density for many storage systems has fallen. In other words, while there is more data being stored in less space, it can be accessed less often.